Engineers use compression (also called source coding or source encoding) to reduce the bit rate of digital video. Compression decreases the cost of storing and transmitting video information by converting the information into a lower bit rate form. Decompression (also called decoding) reconstructs a version of the original information from the compressed form. A “codec” is an encoder/decoder system.
Over the last two decades, various video codec standards have been adopted, including the H.261, H.262 (MPEG-2 or ISO/IEC 13818-2), H.263 and H.264 (AVC or ISO/IEC 14496-10) standards and the MPEG-1 (ISO/IEC 11172-2), MPEG-4 Visual (ISO/IEC 14496-2) and SMPTE 421M standards. More recently, the HEVC standard has been under development. A video codec standard typically defines options for the syntax of an encoded video bitstream, detailing parameters in the bitstream when particular features are used in encoding and decoding. In many cases, a video codec standard also provides details about the decoding operations a decoder should perform to achieve correct results in decoding.
A basic goal of compression is to provide good rate-distortion performance. So, for a particular bit rate, an encoder attempts to provide the highest quality of video. Or, for a particular level of quality/fidelity to the original video, an encoder attempts to provide the lowest bit rate encoded video. In practice, depending on the use scenario, considerations such as encoding time, encoding complexity, encoding resources, decoding time, decoding complexity, decoding resources, overall delay, loss recovery capability, and/or smoothness in playback also affect decisions made during encoding and decoding.
Typically, a video encoder or decoder buffers previously decoded pictures, which the video encoder or decoder can use when encoding or decoding other pictures. Such reconstructed and buffered pictures are often called reference pictures. Some video codec standards describe elaborate rules for managing and updating which reference pictures are buffered, and which reference pictures are no longer used for reference. This can permit an encoder to improve compression efficiency by making good decisions about which reference pictures to use, but the process of managing and updating reference pictures can be complicated for the encoder and decoder. Also, a decoder uses various pieces of information in the bitstream of encoded video data to track and update the state of its reference picture buffer and lists of reference pictures. Loss of information from the bitstream (e.g., due to packet loss or corruption) can adversely affect decoding for a significant period of time if the internal state of the decoder for its reference picture buffer and/or lists of reference pictures deviates from the expected state, and the decoder no longer uses the appropriate reference pictures.